The objectives of the proposed research are to study the effect of the perfusion pressure (systemic BP-IOP), glaucoma medications and other drugs on the oxygen tension of the optic disk and to correlate these effects with optic nerve function. A micro oxygen electrode is placed on the intraocular portion of the optic nerve of cats and rhesus monkeys using a method we have previously described. The intraocular pressure is controlled by an anterior chamber cannula and the blood pressure is continuously monitored with a pressure transducer. The perfusion pressure is decreased by either elevating the IOP or lowering the BP. The arterial hemoglobin saturation is measured by circulating the arterial blood through a small volume cuvette containing a hemoglobin color sensor. The animals are made hypoxic by the inspiration of various mixtures of air and nitrogen.